The ACT Opposition is concerned a prisoner at the Canberra jail has been able to send an offensive email to a news organisation.

The Government says the prisoner managed to breach the internet filter at the Alexander Maconochie Centre (AMC) because many news websites let users send untraceable and anonymous messages.

The prisoner's access to news sites will be blocked while the gap in the system is closed.

However the Opposition's corrections spokesman Jeremy Hanson says he is worried there is a broader problem.

"My concern is that if this has occurred in one instance what other unauthorised emails have been sent out by prisoners previously," he said.

"What ability do they have to send out emails to conduct criminal or threatening behaviour?"

The Opposition says internet access should be totally blocked at the prison if the Government can not guarantee the system will not be abused again.

The Acting Corrections Minister Joy Burch says that is not necessary.

"I think it's a heavy handed response," she said.

"We've closed the sites we consider to be at risk and that we know of, so they've been closed down. Access to email as I've said is around normalising the environment and we hope that inmates have access to the outside world."